The overall goal of this proposed research is to test the effectiveness of ACTIVE MOMS, a 12-week cognitive behavioral physical activity program, delivered as two interventions (semi-intensive structured exercise and minimum-contact physical activity), for increasing physical activity behaviors of women with gestational diabetes mellitus (GDM), and to determine the impact of this program on maternal and fetal outcomes. Because GDM increases maternal and fetal morbidity and it is associated with the onset of type 2 diabetes and obesity later in life for these women and their children, it is critically important that interventions be studied to treat and prevent GDM. Preliminary research has shown that physical activity may be effective at treating GDM; however, this research is limited in scope and no studies to date have 1) intervened with GDM women with a primary objective of increasing their physical activity behaviors, and 2) examined GDM women's physical activity behaviors with a combination of standardized self-report and objective assessments. Also, the research studying the effects of maternal physical activity on psychological correlates and infant outcomes in this population is scant. As a result of early screening and detection for GDM in the standard care setting, women will be identified as having GDM by 17-18 weeks gestation. Participants will be randomized to receive either the structured exercise, lifestyle physical activity, or standard care control condition. All participants will undergo a baseline (about 19 weeks gestation) and two follow-up assessments (about 33 weeks gestation, 6-weeks postpartum) to measure a series of biobehavioral outcomes. The medical records will be obtained for the participants and their offspring to assess pregnancy biomarkers (e.g., glucose tolerance, maternal weight status), pregnancy and delivery complications, and infant birth outcomes. The intervention will be implemented during the second and third pregnancy trimesters (approximately 20-32 weeks gestation). The primary outcome is women's physical activity behaviors (e.g., mean counts/min, energy expenditure, meeting physical activity recommendations). Secondary outcomes include the effects of maternal physical activity behaviors on psychosocial correlates (e.g., physical activity attitude, self-efficacy, social support; depression, quality of life), pregnancy and delivery complications (e.g., excessive maternal weight gain, need for insulin), and infant outcomes (e.g., birth weight). [unreadable] [unreadable] [unreadable]